The present invention is related to inductor current controllers, and more specifically to solenoid current controllers.
It is known that it is generally desirable to implement actuation of a solenoid by providing an initial or pull-in period of time during which a maximum first current is effectively passed through the solenoid inductor to achieve initial actuation of the solenoid. Subsequently, a smaller magnitude solenoid current is implemented so as to maintain actuation of the solenoid wherein this occurs during a subsequent holding or hold period of time. In this manner, the efficiency of the solenoid controller is increased since only the minimum necessary holding current is utilized by the solenoid for maintaining solenoid actuation whereas a high pull-in or actuation current is initially permitted to insure the rapid response of the solenoid to a solenoid actuation control pulse.
Many prior circuits have implemented the general features of the solenoid current control system discussed above. Some of these systems determine the pull-in time, during which a high value of solenoid current can be drawn, by use of a monostable multivibrator. These systems typically utilize additional monostable multivibrators for implementing cycling of the solenoid current about a first high pull-in level and then about a low holding current level. This cycling is implemented by essentially opening and closing the connection between the solenoid coil and a power supply such that during the pull-in period solenoid current varies between maximum and minimum initial high current levels, and during the holding current period solenoid current varies maximum and minimum lower holding current levels. However, the prior systems which utilize monostable multivibrators for determining when such cycling is to occur between the maximum and minimum current levels are not believed to be sufficiently accurate. This is because there is no direct control of the maximum/minimum current levels of the solenoid current, and therefore other circuit parameters can substantially affect the actual solenoid current level regardless of the accuracy of the monostable multivibrators.
While some prior systems have sensed solenoid current directly and have utilized comparators which react to both the sensed solenoid current and current limit reference thresholds so as to control the variation of solenoid current, typically these systems utilize hysteresis to implement the desired switching between maximum and minimum current levels. This use of hysteresis can present stability problems due to temperature variation of the feedback gain which implements the hysteresis. In addition, the use of hysteresis generally results in the reference threshold levels being a function of the sensed solenoid current, and this again is not believed to be a desirable result. Circuit stability would be enhanced if reference thresholds for comparators are fixed during critical operating cycles. In addition, in some systems the pull-in period of time during which a relatively high current for the solenoid is permitted is a function of the sensed solenoid current, and this can lead to variable circuit performance which would be undesired.
In addition to the above disadvantages of prior circuits, typically the circuitry required for implementation of such prior systems is relatively complex, and circuit design flexibility enabling the selection of different maximum and minimum current limits during the pull-in and hold periods is difficult to achieve.